when people update /etc/example/bgpd.conf, the forget to update the

when people update /etc/example/bgpd.conf, the forget to update the
regression test.

- make bgpd-example just check the config file we ship,
to make sure the config is not broken

- make bgpd-printconf checks a config file, writes the parsed output to a file
and checks if that file is parsable config, i.e. this check makes sure that
the config printer produces parsable configuration.

This way we still have to update the regression tests when the syntax
changes, and we fail if the commited example is bad, but we do not fail
just because someone forgot to update a file here.

ok claudio